How can I develop a reinforcement learning agent that plays memory cards game?
The main thing to keep in mind when designing a reinforcement learning agent is that you need to develop an interactive environment in which the agent can learn and define the possible moves the agent can make. In your case, the environment is the memory cards. 

Next you need to define how the agent can interact with the environment, that is choosing a card or flipping a card etc. Also how the environment will behave when the agent chooses a move. 

Finally you need to define the fitness function, in this case the score (number of pairs created) should be fine. You should have a look at some online RL competitions to get a better understanding of the architecture. AWS DeepRacer